Ultrasonic welding is an industrial manufacturing process where high frequency ultrasonic acoustic vibrations can be applied to mating parts held together under pressure to create a solid-state weld. Ultrasonic welding can be preferable to other bonding methods in high volume manufacturing environments due to short weld times and ease of automation. When forming an ultrasonic weld between two parts, it can be common to include an energy director molded into one of the mating parts. An energy director can include a triangular shaped ridge molded into the mating surface of one of the parts. This energy director can limit initial contact between the mating parts to a very small area, and can focus the ultrasonic energy at the apex of the triangular ridge. During the welding process, the concentrated ultrasonic energy can cause the ridge to melt and the melted material to flow throughout a joint area, bonding the parts together.
When forming an ultrasonic weld, it can be advantageous for the entire length of the energy director to impact a mating part at approximately the same time. This can prevent an excessive amount of ultrasonic energy from being concentrated through one portion of the energy director at any time during the welding operation. Excessive energy concentrated in one area can lead to a weaker bond or cause more of the part than the energy director to melt. Sometimes space restraints can require that the two mating parts rotate relative to each other during the ultrasonic welding process. This can pose a problem because a portion of the energy director near the pivot point of the rotation can come into contact with the mating part first. The resulting uneven contact between the energy director and the mating part can result in a weaker bond and can risk damaging the parts being welded.
Therefore, what is desired is a method for ultrasonically welding two parts together where one part must rotate relative to the other part during the welding operation.